White clouds over oceans can stop global warming and bring more rains to the land, according to a recent study. Clouds reflect sunlight and cool the planet and those with bigger droplets tend to be darker than those with tiny droplets for the same amount of water.

“If the droplet number in the clouds is increased, the reflectivity increases and it will increase rainfall on land and also slow down global warming,” said Prof Govindasamy Bala, the lead author of the study from Indian Institute of Science (IISc) Bangalore. “Reducing the droplet size and thus increasing the reflectivity is the basic mechanism behind the proposal for whitening the marine clouds to counteract global warming,” he added.

The new climate modelling study by the IISc in collaboration with the Carnegie Institution, Stanford University, and NASA-AMES, California, suggested that altered atmospheric circulation under the scheme could create a monsoonal circulation and cause the continents to become wetter, not drier, on average.

The study is reported in the latest issue of the journal Climate Dynamics titled ‘Albedo enhancement of marine clouds to counteract global warming: impacts on the hydrological cycle.’

Bala said the number of marine cloud droplets could be increased by spraying tiny droplets of sea water into the marine atmosphere. Salt particles in sea water serve as seeds for many tiny cloud droplets.

“For uniform reductions in sunlight over land and oceans, our earlier modelling work showed that the planet could become drier. However, when you selectively reduce the sunlight over only the ocean surface by whitening only the marine clouds (which we have done in the current model), a monsoonal circulation is triggered which increases the rainfall. We find an increase of 7.5 per cent in the overall water budget over land,” Bala said.

To test the climate consequences, Bala and his co-authors used a computer simulation of the global climate system in which atmospheric carbon dioxide concentrations were set at approximately twice of that of the present day. Cloud droplets over the oceans in the model were reduced to make the clouds more reflective. Clouds over land were unaltered.

“As expected, the whitened clouds reflected more solar radiation and offset the warming effect of the high carbon dioxide levels,” Bala, attached with Divecha Center for Climate Change and Center for Atmospheric and Oceanic Sciences at the IISc, said.

The scientists claimed that the effect of this marine cloud seeding is very different on the planetary water cycle when compared with other sunlight reduction methods such as injecting sulphate aerosols into the stratosphere. In reality, persistent marine clouds are found off the west coasts of the continents and only these clouds can be made whiter by making their droplets smaller.